Porcine reproductive and respiratory syndrome (PRRS) is characterized by abortions, stillbirths, and other reproductive problems in sows and gilts, as well as respiratory disease in young pigs. The causative agent is the PRRS virus, a member of the family Arteriviridae and the order Nidovirales. Two distinct genotypes of the virus emerged nearly simultaneously in North America and in Europe in the late 1980's. PRRS virus is now endemic in nearly all swine producing countries, and is considered one of the most economically important diseases affecting the global pork industry.
Currently, commercial vaccines against PRRS include modified live and killed (inactivated) vaccines. Killed vaccines have been criticized for failing to induce robust immunity against heterologous strains of PRRS virus. Modified live vaccines are attenuated by serial passage in cell culture until virulence is lost. Modified live vaccines elicit broader protection than killed vaccines, but can suffer from a number of safety concerns including residual virulence, spread to non-vaccinated pigs, and genetic reversion to virulence. Because of antigenic changes that take place during the attenuation process, such vaccines can also lose some ability to protect against virulent field strains of PRRS virus. There is a pressing need therefore for new and improved modified live vaccines to protect against PRRS.